Isotope analyses based on optical spectroscopic techniques such as atomic absorption or atomic emission have to date been confined to elements such as Li, B, U, Pb and Hg for which the isotope shifts are larger than or comparable with the Doppler width of the transition (of the order of 1 GHz at 300K). Most elements, however, have isotope shifts of the order of only a few hundred megahertz, which precludes the use of Doppler-limited spectroscopic techniques.
The availability of narrow-band tunable laser sources in recent years has led to the development of a number of very elegant techniques for eliminating Doppler broadening in an atomic vapour, thereby allowing the detailed structure of spectral lines to be investigated. One such technique is Doppler-free saturated absorption spectroscopy, described in Hansch et al, Phys. Rev. Lett. 27,707 (1971). With this technique two counter-propagating beams from a narrow-band tunable dye laser are directed into opposite ends of a vapour cell and the saturating effect of the first beam (the pump) on the transmission of the second beam (the probe) by the vapour is detected as the frequency of the laser is scanned through a selected atomic transition. Since the detected signal originates only from atoms which can interact with both laser beams, i.e. atoms which have zero velocity component along the direction of propagation of the laser beams, a Doppler-free spectrum results.
In conventional saturated absorption experiments, the element to be investigated is usually either in the form of a gas or it is converted to an atomic vapour by thermal evaporation in an evacuated cell. Thus, until recently, saturated absorption spectroscopy had been restricted largely to the rare gases and the more volatile elements, such as the alkali metals and alkaline earths, that can be readily vaporised at moderate temperatures. An alternative approach, first described by Gerstenberger et al, Optics Commun. 31, 28 (1979), is to generate the atomic vapour by cathodic sputtering in a hollow-cathode discharge. With this method the element to be investigated is made the cathode of a hollow-cathode discharge and atoms are ejected from the cathode surface by ion bombardment to form an atomic vapour. The sputtering method of vaporisation has the advantage that it is readily applicable to essentially any element, including highly refractory elements (such as Zr) that are very difficult or impossible to vaporise by conventional thermal means. However, with the sputtering method the atomic vapour is necessarily generated in the presence of a rare gas, and the quality of the Doppler-free spectra is degraded by effects of velocity-changing collisions (VCC). These collisions tend to redistribute the velocities of the atoms over the original Maxwellian distribution and thereby introduce broad background pedestals as well as additional broadening to the narrow Doppler-free components. Such effects can be particularly severe for transitions from a ground (or near-ground) level, because of the long time available for the atoms to undergo VCC.
Several techniques have been devised for reducing the effects of VCC in Doppler-free spectroscopy in hollow-cathode discharges. According to one proposal, polarization intermodulated excitation spectroscopy (Dabkiewicz and Hansch, Optics Commun. 38, 351 (1981)) or polarization spectroscopy is used to probe the orientation of atoms in the vapour, so that those atoms which have undergone VCC no longer contribute to the observed signal if their orientation has been destroyed by the VCC. A second technique (Gough and Hannaford, Optics Commun. 55, 91 (1985)), which has been used in saturated absorption and intermodulated fluorescence spectroscopy, is to operate the hollow-cathode discharge at sufficiently high current density to shorten the lifetime of the lower level, thereby reducing the time available for VCC to take place A further technique (Kroll and Persson, Optics Commun. 54, 277 (1985)) which has been suggested for use in saturated absorption spectroscopy is to choose a suitably high chopping frequency for the laser pump beam such that the phase of the broad pedestal component lags that of the narrow Doppler-free component, thereby allowing the narrow component to be isolated (or partially isolated) by the phase-sensitive detector.